The invention relates to an anti-lock device for fluid operated brakes which comprises a brake diminishing cylinder and a brake diminishing piston which is reciprocable within the cylinder by a variable backstop pressure at one side of the piston in opposition to the thrust of a spring at the other side thereof; the piston being effective when in a first position of adjustment for normal braking to permit unrestricted flow of pressure fluid from a main cylinder provided with an associated actuating element, to at least one wheel brake cylinder; and variation of the backstop pressure under the control of a wheel sensor being effective to move the piston into a second position of adjustment for anti-lock in which it interrupts the flow of pressure fluid to the wheel brake cylinder and enlarges the braking fluid confining space between the brake diminishing piston and the wheel brake cylinder.
An anti-lock device for fluid operated brakes has heretofore been known wherein a brake diminishing piston is urged by the combined pressure of braking fluid and of a spring into a first position of adjustment in which it unblocks a flow passage between a main cylinder and a wheel brake cylinder. When an associated wheel sensor signals an imminent brake lock a backstop pressure is built up in opposition to the pressure of the spring and braking fluid which urges the brake diminishing piston into a second position of adjustment in which the braking fluid passage is blocked and the braking effect is eliminated by enlargement of a braking fluid confining space between the brake diminishing cylinder and the wheel brake cylinder. If, in this case, the control of the backstop pressure fails, as for instance due to a valve for reducing the backstop pressure becoming stuck or due to a malfunctioning of the electrical control system, the pressure fluid passage from the main cylinder to the brake cylinder remains blocked even when a normal brake action is desired by operation of the brake actuating element which is normally a brake pedal. This may result in dangerous driving conditions.
Another anti-lock device for fluid brakes has also heretofore been known wherein a spring loaded check valve is placed into the pressure fluid passage between the main cylinder and the wheel brake cylinder. To provide for normal braking the valve ball of the check valve is raised from its seat by a brake diminishing piston which in turn is urged by a backstop fluid pressure into a first position of adjustment. When a brake lock is imminent the backstop fluid pressure is reduced so that the check valve closes and the brake diminishing piston enlarges the volume between the wheel brake cylinder and the brake diminishing cylinder. If upon disappearance of the locking hazard, restoration of the backstop fluid pressure should fail, normal braking would be impossible because of the closure of the check valve. In order to overcome this disadvantage the mentioned prior art anti-lock is designed so that upon lowering of the backstop fluid pressurs below a predetermined lower limit a spring loaded safety piston moves the brake diminishing piston into its adjusted position for normal braking in which it opens the check valve. With no backstop pressure effective the fluid passage between the main and brake cylinders is thus reopened so that normal braking without anti-lock protection will be possible. The mentioned prior art anti-lock device has the disadvantage of being structurally complicated and mechanically trouble-prone. It requires an individual check valve and a supplemental spring loaded safety piston.